1. Field of the Invention
The present invention relates to a semiconductor device having a MIS (Metal-Insulator-Semiconductor) structure, to which an electric field is applied, such as a field effect transistor, and a method of manufacturing such a semiconductor device.
2. Related Art
In order to meet the requirements of a speed-up and a higher integration of LSIs, the miniaturization of transistors is being further advanced. As this trend is accelerating, a thinner gate dielectric film is continuously being sought after. In a conventional field effect transistor having a MIS structure (MISFET=Metal-Insulator-Semiconductor Field Effect Transistor), SiO2 is used as a material of a gate dielectric film. However, when the thickness of the SiO2 layer is decreased to be as thin as 1 nm, a problem arises in that there is an increase in a leakage current flowing from a gate metal to a substrate via the SiO2 layer. In particular, this problem is serious in a MISFET with a low standby power.
In order to solve this problem, the introduction of a high-k gate dielectric film as a substitute for the SiO2 gate dielectric film is being considered. The advantageous effect of the use of a material having a higher dielectric constant than SiO2 for forming a gate dielectric film is that gate capacitance at a certain level can be secured without decreasing the effective film thickness (actual film thickness or physical film thickness) of the gate dielectric film. As a result, it is possible to suppress the leakage current flowing through the gate dielectric film. However, there is a problem in that a high-k dielectric material tends to generate an interface state, a fixed charge, etc., because generally a high-k dielectric material does not have a good interface characteristic at an interface with a silicon substrate.
Furthermore, a so-called metal silicate, which is a metal-added SiO2, is also being considered to be a material for gate dielectric film. Since a metal silicate contains silicon, the relative dielectric constant thereof is lower, i.e., 8-20. However, a metal silicate is superior in interface characteristic at an interface with a silicon wafer, resulting in that a decrease in drive current capability caused by interface defects, which is easy to occur in the case of a general high-k material, is unlikely to occur.
More strictly speaking, the interface characteristic at an interface between a silicon substrate and a metal silicate layer is far behind that at an interface between a silicon substrate and a SiO2 layer. For example, when a metal silicate is used to form a gate dielectric film of an FET, there is a problem in that electrons flowing through a channel region at a surface of the silicon substrate suffer the influence of remote scattering caused by a potential field generated by the metal contained in the metal silicate. Furthermore, nitrogen added to the metal silicate is also considered to be a cause of the degradation of the interface characteristic.
In order to solve this problem, a so-called “graded composition metal silicate” structure has been proposed to improve the interface characteristic of a dielectric film, as disclosed in Japanese Patent Laid-Open Publication No. 2000-49349. In this structure, the compositions of metal and nitrogen within a metal silicate are lower near the silicon substrate, but become higher as distance increases from the silicon substrate.
Furthermore, a structure is proposed, in which a metal composition is higher at the center of a gate dielectric film and lower near a gate electrode and near the interface with a silicon substrate, as disclosed in Japanese Patent Application No. 2002-49464.
As described above, a structure of gate dielectric film using a metal silicate is proposed, in which metal and nitrogen compositions are graded in order to maintain the interface characteristic at the interface with silicon to be good. However, since the metal or nitrogen concentration at the gate electrode side is higher in the aforementioned structures, the band offset of the dielectric film decreases, which may result in an increase in leakage current.